The present invention pertains generally to a fluid circulation system for the pumping of fluids in a heating system by bubble action. The advantages of solar heating systems relying on the imbalance of fluid heads for circulation are essentially twofold, the avoidance of costly fluid conduit systems with major components located by necessity at an elevated location in a building above a solar collector. Solar heating systems utilizing bubble propelled fluids are disclosed for example in U.S. Pat. Nos. 4,237,866; 4,478,211; 4,340,030 and Re. 31,032. While circulation of fluid by a bubble pumping action is, as noted above, very desirable a problem exists in providing and maintaining adequate bubble generation particularly in a closed system. Over a period of time bubble nucleation sites in a fluid vessel tend to diminish to hinder bubble generation and hence fluid circulation. A discussion of bubble nucleation per se is set forth in U.S. Pat. No. 3,696,861.